I have/had the same problem ! But I think I could solve it now.
Dropbox could not establish a secure connection...
since there were a lot of problems in the last days with thunderbird and other programs,
which could not connect to servers, the problems were caused by eset. disabled/enabled the tls/ssl
option solved the problem.
so I thought it would help with dropbox too, but it didnt ! so I upgraded dropbox the the latest version
as RJanata wrote, but did not work too. so what worked for me :
- TSL/SSL Filter, search and delete all dropbox entrys
- Disabled SSL/TLS Filtering
- started Dropbox (Dropbox syncs)
- enabled SSL/TLS FIltering
- quit dropbox
- started dropbox again... and connection works
